analogmicro.en.cav444...cav444 linear c/v-converter for capacitive input signals may 2014 – rev....

CAV444 Linear C/V-Converter for capacitive input signals

Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz May 2014 – Rev. 3.0

Phone:+49 (0)6131/91 0730-0 Fax: +49 (0)6131/91 073-30 Internet: www.analogmicro.de E–Mail: [email protected]

Principle Function

Capacitance\Voltage-converter IC with linear transfer function and adjustable output

Function

CAV444 is an integrated circuit for capacitive sensing applications. The IC converts the connected input measurement capacitance into an output voltage, which is a linear function of the measure-ment capacitance.

CAV444 can be used as stand-alone analog signal-processing IC or as front-end for a micro proc-essor for electronically calibratable sensor systems.

Typical Applications

• Humidity measurement

• Distance measurement

• Level sensing

• Material identification

• Object detection


May 2014 – Rev. 3.0 /12

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CONTENTS

FEATURES..................................................................................................................................... 3

BLOCK DIAGRAM.......................................................................................................................... 3

GENERAL DESCRIPTION ............................................................................................................. 3

SPECIFICATIONS.......................................................................................................................... 4

1. Electric Specifications ................................................................................................................................ 4

2. Absolute Maximum Values ........................................................................................................................ 5

APPLICATION INFORMATION ...................................................................................................... 6

1. Functional Principle.................................................................................................................................... 6

2. Transfer Function ....................................................................................................................................... 7

3. C/V-converter with adjustable gain and offset ........................................................................................... 8

4. Standard Dimensioning.............................................................................................................................. 9

5. Calibration Procedure ................................................................................................................................ 9

6. Operation Instructions .............................................................................................................................. 10

PACKAGE AND PINOUT.............................................................................................................. 11

DELIVERY FORMS ...................................................................................................................... 11

ACCESSORIES............................................................................................................................ 11

FURTHER LITERATURE.............................................................................................................. 11

NOTES ......................................................................................................................................... 12


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FEATURES

• large measurement capacitance input range: 10pF up to 10nF

• ratiometric voltage output

• linear transfer function

• adjustable offset and gain

• fast response time

• measurement oscillator frequency: 1kHz-240kHz

• integrated temperature sensor

• supply voltage 5V ± 5%

• large temperature range: –40°C...+105°C

• easy-to-use calibration procedure (Excel–sheet)

• available in SO16 or as die

• RoHS conform

BLOCK DIAGRAM

GENERAL DESCRIPTION

CAV444 is an integrated C/V converter suitable for the signal-processing in capacitive sensor sys-tems. Its output voltage is a linear function of the connected measurement capacitance CM and rati-ometric to the supply voltage.

The IC is completely analog leading to a fast re-sponse time and a resolution only limited by the signal to noise ratio.

CAV444 provides the complete electronics needed for the conversion of capacitive input signals into voltage output signals, which can be amplified and offset adjusted using external trimming resistors.

The IC can be used as stand-alone analog signal-processing IC or as front-end for a micro processor for electronically calibratable sensor systems.

An Excel-sheet simplifies the external component’s dimensioning as well as the trimming of complete sensor systems.

Figure 1: CAV444's block diagram

CAV444 Linear C/V-converter for capacitive input signals

May 2014 – Rev. 3.0 /12

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SPECIFICATIONS

1. Electric Specifications

Tamb = 25°C, VCC = 5V, ICM = ICW = 20 µA (unless otherwise noted)

Parameter Symbol Conditions Min. Typ. Max. Unit

System

Operating Temperature Tamb –40 105 °C

Supply Voltage VCC 4.75 5.00 5.25 V

Current consumption

ICC Tamb = 25°C, G = 1 2 mA

Tamb = –40 ... 105°C, G= 1 2.2 mA

Measurement Capacitance1)

CM 10 10000 pF

Output Voltage2)

VOUT 1.0 4.0 V

Temperature Coefficient VOUT3)

TCVOUT dVOUT /(dT·VSpan) @Tamb = –40 ..85°C G = 1, CW = 1.5 nF, CM = 0.1 ..1nF

±100 ppm/°C

Maximum Input Signal Fre-quency

4)

fSIG,max @ CM= 10pF, RCM = 125kΩ, CF1 = 2nF, CF2 = 2nF

3.5 kHz

Minimal Response Time5)

tRES,min @ CM= 10pF, RCM = 125kΩ CF1 = 2nF, CF2 = 2nF

0.4 ms

Measurement Oscillator

Oscillator Frequency Range fM fM= VREF / (2* DVCM* RCM *CM ) 1 240 kHz

Oscillator Voltage DVCM 2.1 2.15 2.2 V

Oscillator Current Resistor RCM 50 125 kΩ

Oscillator Charge Current ICM ICM= VREF / RCM 20 50 µA

Oscillator Charge Current Spread ICM @RCM = 125kΩ 19 20 21 µA

f/V-Converter

Converter Capacitor Range CW CW = 1.4*CM,max *RCM / RCW

@RCW =125kΩ, RCM = 125kΩ

14 14000 pF

Minimum Converter Voltage VCW,min @PIN 16 1.15 1.2 1.25 V

Maximum Converter Voltage VCW,max @PIN 16 4.1 V

Converter Current Resistor RCw 100 1250 kΩ

f/V-Capacitor Charge Current ICW ICW= VREF / RCW 2 25 µA

f/V-Capacitor Charge Current Spread

ICW @RCW = 125kΩ 19 20 21 µA

f/V-Stage Biasing Resistor RA RA= 0.48* RCW , @RCW =125kΩ 60 kΩ

Lowpass Stage

Internal Low Pass Resistors R01, R02 20 kΩ

Low Pass Filter Capacitor6)

CF1 ,CF2 2 1100 nF

Corner Frequency 1 (3 dB) fCF1 @CF1 =2nF 4 kHz

Corner Frequency 2 (3 dB) fCF2 @CF2 =2nF 4 kHz

Offset Voltage LP-Stage VofsLP

-2 2 mV


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Output Stage

Adjustable Gain G 1 10

Output Current IOUT Source, Sink7) -100 100 µA

Differential Output voltage VDIFF VDiff =Vout - VREF -1.5 1.5 V

Resistive Load at Pin VOUT RL 40 kΩ

Capacitive Load at Pin VOUT CL 100 pF

Input Offset Voltage VOFS RL = 100MΩ -2 +2 mV

Voltage Reference VREF

Reference Voltage VREF 2.49 2.5 2.51 V

VREF vs. Temperature TCVREF dVREF /(dT·VREF), Tamb = –40...+85°C ±50 ppm/°C

Output Current IVREF Source, Sink7) -100 100 µA

Load Capacitance CVREF 80 100 120 nF

Temperature Sensor VTEMP

Output Voltage VTEMP @ Tamb = 25°C, RLoad ≥ 50MΩ 2.20 2.32 2.45 V

Sensitivity S S = dVTEMP/dT, RLoad ≥ 50MΩ 8 mV/°C

Resistive Load RLoad 10 MΩ

Thermal Nonlinearity RLoad ≥ 50MΩ, Tamb = –25 ... 85°C 0.5 %

Notes:

1) For linearity better than 0.3% (BFSL) it is recommended to use a maximum measurement capacitance CM,max not larger than ten times the particular minimum measurement capacitance CM,min.

2) If VCC ≠ 5 V, the maximum of VOUT is given by 0.8·VCC.

3) The temperature coefficient is normalized with VSpan = VOut(CM, max) – VOut(CM, min). TCVOUT is increased, if CM (CW) is decreased.

4) The maximum input signal frequency fSIG,max is defined as the measurement capacitance’s change rate, at which the low pass filter reduces the output voltage by 6 dB. In general fSIG,max depends on the choice of values of CF1 and CF2 and can be increased if CF1 and CF2 are decreased (see note 6 below).

5) CAV444’s response time can be decreased if smaller low pass capacitors CF1 and CF2 are chosen.

6) The typical dimensioning of the low pass capacitors CF1 and CF2 is based on the requirement that a ripple of less than 1 ‰ remains at the highest oscillator frequency. Smaller capacitances can be chosen but lead to a higher ripple.

7) Currents flowing into the IC have a negative sign.

2. Absolute Maximum Values

Parameter Symbol Condition Min. Typ. Max. Unit

Maximum Supply Voltage VCCmax 6 V

Maximum Oscillator Charge Current ICMmax 50 µA

Maximum f/V-Capacitor Charge Current ICWmax 25 µA

Storage temperature T -55 125 °C

ESD Susceptibility1)

VESD HBM 2 kV

Notes:

1) ESD Protection on all pins except pin 12, pin 14 and pin 16.

Table 1: CAV444’s electric specifications

Table 2: Absolute Maximum Values


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APPLICATION INFORMATION

1. Functional Principle

CAV444’s functional principle is described using Figure 2, where the functional blocks of the IC, the signal patterns inside the IC and a basic external wiring with necessary passive components are shown.

The IC consists of six functional blocks: the measurement oscillator, the f/V converter, the low pass filter, the output stage, the temperature sensor and the power supply. The power supply block drives all the other blocks and also generates a reference voltage of 2.5 V. The temperature sensor block provides an output voltage VTEMP proportional to the IC’s temperature. The capacitance measurement path consists of the re-maining four blocks and is described below.

CAV444 uses the following measurement principle: The input measurement capacitance CM, connected at pin 12, works as the capacitor of the measurement oscillator block. CM is charged and discharged periodi-cally with constant current by CAV444. At the measurement oscillator’s output a triangular voltage with con-stant amplitude is generated. The frequency of this triangular voltage (measurement oscillator frequency fM) depends on the connected measurement capacitance CM. Using the f/V converter block and the following low pass filter block the triangular voltage is converted into a DC voltage, which can be impedance trans-formed or amplified at the output stage. Furthermore the output signal’s offset can be adjusted by using the circuit in Figure 4. The output voltage VOUT at pin 5 (referenced to GND) is ratiometric to the supply voltage and a linear function of CM.

1

1 If the differential output voltage VDIFF= VOUT - VREF is evaluated and if no further offset adjustment is done, VDIFF is di-

rectly proportional to CM.

Figure 2: CAV444 with signal path and a basic circuit


May 2014 – Rev. 3.0 /12

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2. Transfer Function

The system shown in Figure 2 can be used as basic circuit for the CAV444. The output voltage VOUT at pin 5 (referenced to GND) is given by:

REFM

CWW

CMCM

REFMLPASMOUTVC

RC

RVVCVCV +⋅

⋅⋅

⋅∆⋅=+=

8

3)()( (1)

with

VLPAS = low-pass voltage VREF = reference voltage (with VREF = 2.5 V) CW = f/V converter capacitance CM = measurement capacitance ∆VCM = voltage amplitude at the measurement capacitance (with ∆VCM ≈ 2.1 V) RCM = oscillator current resistor

RCW = reference current resistor

RCM and RCW define the charge currents ICM (=VREF/RCM) and ICW (=VREF/RCW) for the input capacitance CM

and the converter capacitor CW. A typical dimensioning for these fixed resistors is RCM = RCW = 125 kΩ.2

CW’s dimensioning is given by:

CW

MCM

WR

CRC max,4.1

⋅⋅= (2)

wherein CM,max is the particular used maximum measurement capacitance. Combining (1) and (2) leads to

REF

M

M

MOUTV

C

CCV +⋅=

max,16

9)( (3)

The output voltage given by equation (3) is a linear function of CM and is illustrated in Figure 3. A minimum measurement capacitance CM,min ≠ 0 leads to a minimum output voltage VOUT(CM,min) > VREF.

If instead of VOUT the differential voltage VDIFF between pin 5 and pin 6 is evaluated, the output signal is di-rectly proportional to CM.

max,16

9)()(

M

M

REFMOUTMDIFFC

CVCVCV ⋅=−= (4)

2 For an optimal and easy dimensioning of the external passive components Analog Microelectronics has developed the

Excel-tool, Kali_CAV444 (see section “Further Literature”).

Figure 3: Output signal VOUT as a function of the measurement capacitance CM


May 2014 – Rev. 3.0 /12

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3. C/V-converter with adjustable gain and offset

For the adjustment of the output signal’s offset and gain five resistors R1 .. R5 have to be added to the net-work in Figure 2. The complete measurement circuit with CAV444 is illustrated in Figure 4.

For the capacitive measurement system in Figure 4 the output voltage VOUT at pin 5 (referenced to GND) is given by:

REFLPASOUTVBVGV ⋅+⋅= (5)

with

535443

5453

5

1

2

11RRRRRR

RRRR

R

R

R

RG

++

+⋅++= and

535443

5453

5

11RRRRRR

RRRR

R

RB

++

−⋅+= . (6)

wherein R2, R4 and R5 are fixed resistors, R1 is the trimming resistor for the adjustment of gain and R3 is the trimming resistor for the adjustment of offset.

3

Using the equations (1), (5) and (6) it is easy to see that the output voltage is again a linear function of the measurement capacitance CM.

REFM

CWW

CMCM

MOUTVBC

RC

RVGCV ⋅+⋅

⋅⋅

⋅∆⋅⋅=

8

3)( (7)

With the standard dimensioning for CW (see equation (2)) the output voltage is given by:

REF

M

M

MOUTVB

C

CGCV ⋅+⋅⋅=

max,16

9)( (8)

3 The final values of R1 and R3 depend on the desired output voltage as well as on the individual sensor setup. In addi-

tion variances of the components (like manufacturing tolerances in RCM, RCW, CW, CM…) and the parasitic capacitances also influence the value of R1 and R3 (see section “Calibration Procedure”).

Figure 4: CAV444 used as C/V-converter with adjustable gain and offset


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4. Standard Dimensioning

A basic dimensioning of the passive external components in Figure 4 with a measurement capacitance not larger than 1 nF is given in the table below. For the application specific capacitance ranges an optimal di-mensioning can be calculated using the Excel-sheet Kali_CAV444.

Parameter Symbol Min. Typ. Max. Unit

Output Stage Resistors (1%) R2 , R4 , R5 100 kΩ

Gain Resistor (0.1%) Calibration Start Value1)

R1 33 kΩ

Offset Resistor (0.1%) Calibration Start Value1)

R3 100 kΩ

f/V-Stage Biasing Resistor RA 60 kΩ

Measurement Oscillator Current Resistor RCM 125 kΩ

f/V-Stage Current Resistor RCW 125 kΩ

Low Pass Filter Capacitors (depends on value of CM,max)3)

CF1 ,CF2 1000 nF

Reference Voltage capacitor (VREF = 2,5V)4)

CVREF 80 100 120 nF

f/V converter capacitor CW 1.4 * CM,max pF

Notes:

1) The given values for the trimming resistors R1 and R3 are the initial values at the start of the trimming process. During the trimming process R1 and R3 will be set to individual values (calculated by the Excel-sheet Kali_CAV444).

2) In many applications it is sufficient to use standard components, like e-series capacitors and resistors with low temperature coefficients (≤100 ppm). For RCM, RCW, RA, CVREF and CF1/F2 a variance of 5% of the given value is acceptable without a decrease in performance. For components, which have to have lower variances, the toler-ances are given in round brackets in Table 3.

3) CF1 und CF2 depend on the minimum measurement capacitance CM,min and are calculated in such a way, that only a ripple of less than 1 ‰ remains at the highest oscillator frequency fM.

4) For best performance a high grade ceramic capacitor has to be used for CVREF.

5. Calibration Procedure

During the design of capacitive sensor systems it always has to be taken into account that there are parasitic capacitances beside the measurement capacitance, which influence the output signal and the calibration process. To simplify the consideration of these effects and to assist the user during the calibration of sensor systems based on the circuit in Figure 4, Analog Microelectronics developed a calibration strategy, imple-mented in the Excel-sheet Kali_CAV444. This Excel-sheet is available at www.analogmicro.de.

Using the Excel-sheet Kali_CAV444 the calibration of capacitive sensor systems with CAV444 is done in two steps, a dimensioning and a trimming step. At first a dimensioning of the passive external components is cal-culated for the specific type of sensor implemented with CAV444. This dimensioning is mostly dependent on the capacitive measurement range of the sensor. Only the minimum and the maximum measurement capaci-tances as well as the desired differential output voltage and the charge currents ICM and ICW are needed for the calculation. Based on this input values the program calculates a suitable dimensioning of the external components, which can be used to assemble this type of sensor system.

After the specific sensor systems have been built with the calculated dimensioning, a trimming of offset and gain (using the trimming resistors R1 and R3) of the individual sensor systems has to be done. Therefore the output voltage has to be measured at the minimum and the maximum measurement capacitance. Based on these values the Excel-sheet calculates the final trimming resistor values for R1 and R3. After trimming R1 and R3 to the calculated values the sensor is ready for operation and all parasitic capacitances as well as the tolerances of the used components are taken into account.

Table 3: Standard values for external components at ICM = ICW = 20 µA, CM,max < 1nF


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6. Operation Instructions

It is recommended to do the external component’s dimensioning using the Excel-sheet Kali_CAV444. In general the absolute maximum values in Table 2 should not be exceeded. For first investigations Analog Mi-croelectronics offers the BBCAV444, a pre-assembled breadboard (see accessories) with easily adaptable measurement capacitance ranges, which can be used to study the behavior of capacitance sensor heads as well.

To realize the dimensioning calculated by the Excel-sheet networks of not more than two resistors or capaci-tors should be used. In many applications it is sufficient to use standard components, like e-series capaci-tors, which fit to the calculated values within 5% (see section “Standard Dimensioning”)

For the PCB layout it is recommended to place CM and CW as close to the IC as possible and to keep con-ducting lines from the IC to its external passive components short, leading to small parasitic capacitances.

The parasitic capacitances inside the IC and in the sensor setup enlarge the used capacitances, especially the measurement and converter capacitance. This influences the capacitance measurement directly. There-fore it is important to keep the parasitic capacitances stable (e.g. no flexible wiring).

A capacitor can be added in parallel to the measurement capacitance to measure capacitances smaller than the specified 10 pF. But in some cases the parasitic capacitances in the measurement setup are already large enough, making the additional capacitance unnecessary. Adding a capacitance in parallel to the meas-urement capacitance can also be advantageous if the ratio of the used maximum and minimum measure-ment capacitances CM,max/CM,min is larger than ten to increase CAV444’s linearity.

In real sensor applications the temperature behavior is mainly influenced by CM’s, CW’s and further external component’s temperature coefficients. In general an optimal temperature behavior can be achieved by using CM and CW with equal temperature coefficients (TC). Because CAV444’s temperature coefficient increases for small measurement capacitances it can be useful to shift the measurement capacitance range to higher values by connecting an additional capacitor in parallel to the measurement capacitance.

In general ESD precautions are necessary during assembly and handling of the device. It is essential to ground machines and personnel properly.

Notes:

1. If the voltage signal at pin 12 or 16 is measured, the probe’s capacitance is added to the capacitance at the spe-cific pin, which changes the output voltage signal. If measured at pin 12 the oscillator frequency is decreased and if measured at pin 16 the signal amplitude at the f/V-converter is reduced. To check the frequency and the signal amplitude after the f/V-converter without influencing the output signal it is possible to remove the low pass capaci-tor CF1 and measure at pin 15.

2. For level sensing applications it is important to isolate the measurement electrodes from a conductive medium. Otherwise the output voltage is affected by the conductivity of the medium.


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PACKAGE AND PINOUT

The CAV444’s standard packaging is a SO16 (n) package (for dimensions please see the packaging catalog http://www.analogmicro.de/products/analogmicro.de.en.package.pdf ).

DELIVERY FORMS

CAV444 is available as:

ORDER NUMBER DELIVERY FORM

CAV444-0-SO16 CAV444 inside SO16 packaging

CAV444-WAF sawn wafer on 8“ blue foil

CAV444-Adapt CAV444 soldered to a SO16-DIL16 adapter

ACCESSORIES

To support developments using CAV444 Analog Microelectronics GmbH offers the Breadboard BBCAV444.

ORDER NUMBER DELIVERY FORM

BBCAV444 BBCAV444 - BreadBoard (PCB with CAV444)

FURTHER LITERATURE

1. Calibration tool Kali_CAV444.xls (http://www.analogmicro.de/german/products/cav444.htm)

2. Package catalog (http://www.analogmicro.de/products/analogmicro.de.en.package.pdf)

3. Application Notes on CAV444 (on request)

4. CAV444 - Die Size and Padout (on request)

PIN NAME DESCRIPTION

1 RCM oscillator current adjustment

2 RCW f/V converter current adjustment

3 VB bias voltage connected to VCC

4 GAIN gain adjustment

5 VOUT output voltage

6 VREF reference voltage ca. 2.5V

7 VTEMP temperature sensor voltage output

8 N.C. not connected

9 N.C. not connected

10 GND IC ground

11 VCC supply voltage

12 CM measurement capacitance

13 CF2 2nd

order lowpass capacitor

14 RA stabilizing resistor f/V converter

15 CF1 1st order lowpass capacitor

16 CW f/V converter capacitor

Table 4: Pin assignment CAV444 SO16 Package

Figure 5: Pinout CAV444 SO16


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NOTES

Analog Microelectronics GmbH reserves the right to amend any dimensions, technical data or other information contained herein without prior notification.

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